When you have a good domain but don't know what to use it for

On many occasions, I will get a good name for a domain but not really know what to use the domain for. Maybe the domain will be niche related but actually investing time to develop that type of site would take so much more time than available. For those situations, I'm never sure if it's best to go ahead and purchase the domain and hope something comes of it or just let the domain name pass.